Evaluation of Inorganic Phosphate Solubilizing Efficiency and Multiple Plant Growth Promoting Properties of Endophytic Bacteria Isolated From Root Nodules Erythrina Brucei

Background: The majority of phosphorous in the soil is fixed and unavailable to plant nutrition, hence in scarcity. Phosphate solubilizing bacteria, the ecological engineers, are considered as the best, sustainable and eco-friendly options. The objectives of this study were to screen and evaluate inorganic phosphate solubilizing efficiency and assess multiple plant growth promoting traits of E. brucei root nodule bacterial endophytes.Results: A total of 304 nodule bacterial endophytes were screened for phosphate solubilization potential on solid PA medium among which 119 (39%) were potential tricalcium phosphate solubilizers. None of these isolates were able to form clearly visible halos on aluminum phosphate (AlPO4), Al-P or iron phosphate (FePO4), Fe-P supplemented PA medium. Out of 119 inorganic phosphate solubilizing endophytes, 40.3% were IAA producers. Based on phosphate solubilization index, the potential bacterial endophytes were identified to Gluconobacter cerinus, Acinetobacter soli, Achromobacter xylosoxidans and Bacillus thuringiensis using the 16S rRNA gene sequences analysis. All the selected isolates were potential solubilizers of the three inorganic phosphates (Al-P, Fe-P and tricalcium phosphate, TCP) included in liquid NBRIP medium. The highest values of solubilized TCP were recorded by isolates AU4 and RG6 (A. soli), 108.96 mg L-1 and 107.48 mg L-1, respectively at sampling day3 and 120.36 mg L-1 and 112.82 mg L-1, respectively at day 6. The highest values of solubilized Al-P and Fe-P were recorded by isolate RG6, 102.14 mg L-1 and 96.07 mg L-1, respectively at sampling days 3 and 6, respectively. The highest IAA, 313.61µg mL-1 was recorded by isolate DM17 (B. thuringiensis). These selected potential isolates were also HCN, NH3, and hydrolytic enzymes producers. The isolates were also varied in tolerance to eco-physiological stressors and exhibited versatility to carbon and nitrogen substrate utilization. Conclusions: The genera and species Gluconobacter cerinus, Acinetobacter soli, Achromobacter xylosoxidans and Bacillus thuringiensis are the first reports from E .brucei root nodules and Gluconobacter is also the first report to the science as phosphate solubilizer. Isolates AU4 and RG (A. soli) could be potential bio-inoculant candidates for the growth enhancement of the host plant for better agro-forestry practices in acidic and alkaline soils in Ethiopia.